having trouble opening maridDB

This is the error I get due to line 79. 

A PHP Error was encountered
Severity: Compile Error
Message: Cannot use temporary expression in write context
Filename: config/database.php
Line Number: 79


what does this mean? This is line 79
$active_group = 'default';
$query_builder = TRUE;
$active_record=TRUE;
//Linux server?

db['default'] = array(      <<<<<<<<<this is line 79 in database.php
    'dsn' => '',
    'hostname' => 'docker-substantiator_mariadb_1',
    'username' => 'root',
    'password' => 'root',   
    'database' => 'substantiator',
    'dbdriver' => 'mysqli',
    'dbprefix' => '',
    'pconnect' => FALSE,
    'db_debug' => (ENVIRONMENT !== 'production'),
    'cache_on' => FALSE,
    'cachedir' => '',
    'char_set' => 'utf8',
    'dbcollat' => 'utf8_general_ci',
    'swap_pre' => '',
    'encrypt' => FALSE,
    'compress' => FALSE,
    'stricton' => FALSE,
    'failover' => array(),
    'save_queries' => TRUE
);

proof that an old dog can learn new tricks

db['default'] should be $db['default']

Simpler is always better